FIG. 1 shows the transmission of data in a wireless system according to the state of the art. Several transceivers belonging to the same wireless local area network (WLAN) use the same data transmission channel by means of time sharing. At any specific time only one transceiver is transmitting. Accordingly the transmissions from each transceiver are burst like. For helping the receiving transceiver to identify a data transmission burst and for extracting the delivered information data the transmitting transceiver sends a predefined preamble signal preceding the data portion of the data transmission burst. The transceiver that receives the data transmission burst comprises a preamble detection unit that identifies the preamble and thus identifies the data transmission burst. The transceiver uses further the preamble for estimating data transmission and channel parameters such as channel response and carrier and timing offsets that are needed for the data information extraction.
Commonly several communication networks share the same data transmission media. Specifically collocated wireless networks utilize the same frequency spectrum.
FIG. 2 shows two collocated wireless networks according to the state of the art.
Wireless local areas networks (WLAN) represent a new form of communications among personal computers or other devices that wish to deliver digital data. A wireless network is one that does not rely on cable as the communications medium. Whether twisted pair, coax, or optical fibers, hard wiring for data communication systems within a building environment is expensive and troublesome to install, maintain and to change. To avoid these disadvantages wireless networks transmit data over the air using signals that cover a broad frequency range from few MHz to a few terahertz. Depending on the frequency involved wireless networks comprise radio wireless networks, microwave wireless networks and infrared wireless networks.
Wireless networks are used mainly for connecting devices within a building or connecting portable or mobile devices to a network. Further applications are keeping mobile devices in contact with a data base and ad hoc networks for example in committee or business meetings.
Wireless local area networks (WLAN) and wireless personal area networks (WPAN) are used to convey information over relatively short ranges. A wireless personal area network (WPAN) is defined in the IEEE 802.15.3 standard.
In many situations and scenarios several wireless local area networks (WLANs) are operated simultaneously with each other in the same local area. A typical situation would be a big office wherein many office cubicles are located belonging to different divisions of the same company, e.g. search division, accounting division, marketing division. The computers of each division are connected in such a situation by means of separate wireless local area networks (WLANs). A wireless local area network (WLAN) comprising several transceivers is referred to as a Piconet.
FIG. 2 shows typical scenario where two wireless local area networks (WLANs) are operated in the same local area.
In the shown example a first transmitting transceiver A2 transmits data to a receiving transceiver A4 of the first wireless local area network WLANA on the data transmission channel of the wireless local area network WLANA. Further a transmitting transceiver B3 of the second wireless local area network WLANB transmits data to a receiving transceiver B1 of the same wireless local network WLANB on the data transmission channel of this wireless local area network. The data exchange between transceivers is performed half duplex, i.e. a transceiver can either send or receive data over a data link to another transceiver of the same wireless local area network. The data are exchanged via data packets.
Each Piconet WLANi has its respective data transmission channel, i.e. the data transmission channel is used by all transceivers of the corresponding Piconet WLANi.
In most cases the frequency resources available for a wireless local area network WLAN are bounded by regulations. Usually a certain frequency band is allocated for the wireless local networks. Within this frequency band each transceiver is required to radiate no more than a specified average power spectral density (PSD).
To operate several wireless local area networks simultaneously several proposals have been made.
In frequency division multiplexing (FDM) systems according to the state of the art the allocated frequency band is divided into several sub-frequency bands. In FDM-system each data transmission channel and consequently each Piconet is using a different frequency sub-band. Thus, data transmission in different Piconets (WLANs) can simultaneously be performed without interference.
The disadvantage of FDM-systems is that the available capacity for each Piconet is reduced compared to the case where any Piconet is allowed to use the entire allocated frequency band.
The channel capacity is given by the following formula:
  cap  =      ∫                  log        ⁡                  (                      1            +                                          PSD                ⁡                                  (                  f                  )                                                            N                ⁡                                  (                  f                  )                                                              )                    ⁢              ⅆ        f            
The capacity of each Piconet is larger if it will be allowed to use the full frequency band instead of just the allocated frequency sub-band The reduction in the capacity in FDM-systems translates directly to throughput reduction. Consequently the achievable data bit rate for any specific transmitter-receiver distance is reduced in FDM-systems.
In a CDMA-DSSS (Code Division Multiple Access—Direct Sequence Spread Spectrum) system according to the state of the art a direct sequence spread spectrum is used as a modulation scheme. In DSSS a sequence of many short data symbols is transmitted for each information symbol. In order to support several data transmission channels or Piconets different data sequences with low cross correlation between them are used for different data transmission channels.
In a CDMA-DSSS-system each channel can use the entire frequency band until the maximum possible throughput can be achieved. If some Piconets are working in the same area then the transmission of one Piconet is seen as additional noise by the other Piconets.
The disadvantage of the CDMA-DSSS-System is that there exists a so called near-far problem. When a transceiver in one Piconet is transmitting this transmission will be seen as additional noise by other Piconets. The level of the additional noise is proportional to the cross correlation between the spreading sequences and the received power level of the interferer's signal. For example if the interfering transceiver of Piconet A is close to a receiving transceiver of Piconet B, i.e. closer than a transmitting receiver of Piconet B then the added noise level that the receiving transceiver of Piconet B sees causes a significant reduction in the achievable bit rate for the receiver, so that even a complete blocking of the data transmission channel can occur.
A further proposal according to the state of the art to operate several wireless local area networks (WLANs) simultaneously is to use a CDMA-PH(Code Division Multiple Access—Frequency Hopping)—System. In this CDMA-PH-System the original frequency band is divided into several sub-frequency bands. Any transmitting transceiver uses a certain frequency sub-band for a certain time interval and moves then to the next frequency band. A predefined frequency hopping sequence controls the order of sub-frequency bands such that both the transmitting and receiving transceiver has the information when to switch to the next frequency and to what sub-frequency band.
In a conventional CDMA-PH-System the different data transmission channels are assigned with different frequency hopping sequences.
FIG. 3A shows a CDMA-PH-System according to the state of the art with data transmission channels. A CDMA-PH-System with four data transmission channels can operate four Piconets or wireless local area networks (WLANs) simultaneously at the same local area. In the shown example any transceiver uses a certain frequency band for a transmission interval for 242 ns, remains idle for a predetermined guard time of 70 ns and uses the next frequency band within the next transmission interval etc.
The frequency hopping sequence is fixed for any data transmission channel A, B, C, D. In the given example data transmission channel A has the frequency hopping sequence abc, channel B has the frequency hopping sequence acb, channel C has the frequency hopping sequence aabbcc and channel D has the frequency hopping sequence aaccbb.
A collision is a situation when two transceivers use the same frequency band at the same time. For example a collision between data transmission channel A and data transmission channel B occurs during the first transmission interval when both channels A, B use frequency fa and during the fourth transmission interval when both channels A, B use again frequency fa. A further collision is for example between channel B and channel D during the first transmission interval when both channels B, D use frequency a and the sixth transmission interval when both channels B, D use frequency fb.
When the frequency hopping order of two wireless networks differs two transceivers that belong to different wireless local area networks can transmit at the same time. It may happen that both transceivers use the same carrier frequency at the same time.
One possible CDMA-FH solution is based on OFDM and is called Multiband OFDM. In this case the transceiver transmits a single OFDM in one band and then hops to the next band for transmitting the next OFDM symbol. FIG. 3A depicts 6 OFDM symbols for each channel.
As shown in FIG. 3A the Multiband OFDM Transceiver performs in a time frequency interleaving (TFI) mode band-hopping wherein in each frequency band an OFDM symbol is transmitted. The band-hopping sequence is defined by a TFC code (Time frequency code) stored in a memory. Different collocated networks use different TFC codes. This enables simultaneous transmission of different networks OFDM symbols from collocated networks collide. In common scenarios the collision level enables efficient communication. Yet in some cases the collisions situation is severe and the communication is not efficient. To overcome severe collisions between transmission of different networks frequency domain separation (known as FDM) between the wireless networks can be implemented. This is achieved by adding TFC codes with constant band usage (fixed frequency bands). Accordingly a Multiband OFDM Transceiver according to the state of the art is switchable between a time frequency interleaving mode (TFI mode) and a fixed frequency interleaving mode (FFI mode). FIG. 3B shows 7 channels (7 TFC) where 4 channels are of TFI type and 3 channels are of FFI type.
As can be seen in FIG. 3B the transceiver occupies in the TFI mode three frequency bands, wherein each frequency band has a predetermined frequency bandwidth.
According to the evolving multiband OFDM standard the period of one OFDM symbol is 312.5 nSec, i.e. a data length of 242.5 nSec (128 samples at 528 Msps) and a silence time of 70 nSec (37 samples at 528 Msps) between two transmissions. Consequently the OFDM symbol rate RS=3.2 MHz=1/312.5 nSec. When using three frequency bands there are seven possible time frequency codes (TFC). The first four TFC codes define the frequency band hopping sequence when the transceiver is in the TFI mode. When the transceiver is switched to the FFI mode the transceiver transmits the signal in a fixed frequency band. As shown in the following table and in FIG. 3B, the fifth TFC code indicates that transceiver transmits a signal in a first frequency band, the sixth TFC code indicates that the transceiver transmits the signal in a second frequency band and the seventh TFC code indicates that the transceiver transmits a signal in a third frequency band.
The following TFC code have three frequency bands as summarized in the following table:
TABLE 1TFC IndexCodeType1[1, 2, 3]TFI2[1, 3, 2]TFI3[1, 1, 2, 2, 3, 3]TFI4[1, 1, 3, 3, 2, 2]TFI5[1]FFI6[2]FFI7[3]FFITFC indices 1-7 in the table corresponds to channels A-G in FIG. 3b.
A single burst of transmission is called a PLCP frame. FIG. 4 shows the data format of a PLPC frame used by a multiband OFDM transceiver. Each frame consists of a preamble, a header and a payload data section. The PLPC header is transmitted with a constant data rate of 39.4 Mbit per second whereas the payload data is transmitted with different data rates varying between 53.3 Mbit per seconds and 480 Mbit per second depending on the selected operation mode of the OFDM transceiver. The PLCP frame as shown in FIG. 4 consists of a plurality of OFDM symbols, wherein each OFDM symbol consists of a predetermined number (NCBPS) of encoded data bits. Each OFDM symbol comprises for instance 100 or 200 encoded data bits depending on the selected data rate. As can be seen from FIG. 3B each OFDM symbol is transmitted within different frequency bands fa, fb, fc according to a predetermined frequency hopping pattern. For example three frequency bands fa, fb, fc, are employed by the OFDM transceiver so that seven different frequency hopping patterns are possible as shown in FIG. 3B via a corresponding number of data transmission channels A, B, C, D, E, F, G. Each frequency band fa, fb, fc employed by the OFDM transceiver comprises a center frequency around which a predetermined number of sub-carriers or tones are provided. A frequency comprises for instance 122 sub-carriers consisting of pilot sub-carriers, guard sub-carriers and data sub-carriers. Each sub-carrier is equidistant to its neighboring sub-carrier and can be modulated separately.
For increasing the performance of the data transmission interleaving is employed by an OFDM transceiver.
In a conventional OFDM transceiver according to the state of the art the encoded bitstream is interleaved prior to modulation by a bit interleaving circuit comprising two stages. The conventional bit interleaving unit comprises a symbol interleaving unit and a tone interleaving unit.
The symbol interleaving unit permutes the bits of the received bitstream across different OFDM symbols to exploit frequency diversity across the sub-frequency bands fa, fb, fc. The tone interleaving stage of the bit interleaving circuit according to the state of the art permutes the bits received by the symbol interleaving stage across different data tones (data sub-carriers) within an OFDM symbol to exploit frequency diversity across tones so that more robustness against narrow band interference signals and against frequency-selective channels is provided.
However, the bit interleaving circuit according to the state of the art as employed in a conventional multiband OFDM transceiver has several disadvantages.
In some cases there exists a correlation between the qualities of tones, i.e. sub-carriers, from two different OFDM symbols which have the same index within the OFDM sequence.
A first case where such a correlation exists is when, as in the FFI mode, i.e. channel E, F, G as shown in FIG. 3b, and TFC index 5, 6, 7, and the table above, no frequency band hopping is performed.
A second case is as in channel C, D shown in FIG. 3b where two adjacent OFDM symbols might be transmitted in the same frequency band. For instance the first two OFDM symbols are transmitted in a frequency band fa as shown in FIG. 3B. The next two OFDM symbols are transmitted in the frequency fb as shown in FIG. 3B. The quality of sub-carriers or tones within such a frequency band are correlated to each other so that the quality of interleaving is degraded.
Even in a TFI mode where the frequency band is changed with each OFDM symbol such as in channel A, channel B shown in FIG. 3B there might be a correlation between different tones within one OFDM symbol because the same tones or sub-carriers within one frequency band can be degraded in the baseband in a similar manner. The OFDM symbol is transmitted within a frequency band which comprises for example 122 sub-carriers. The tones or sub-carriers which are located at the edges of the frequency band may suffer more from imperfect filtering than tones which are located closer to a center frequency of said frequency band. The sub-carriers or tones which are located at the edges of the frequency band might even be cut off by misadjusted filters within the baseband transmission. Another possibility is when zero-IF conversion is used so that the DC offset effects the near-DC tones within the frequency band.
Since conventional interleaving with a bit interleaver circuit according to the state of the art is not optimal in all the above mentioned cases the performance of the data transmission of an OFDM transceiver which employs a conventional bit interleaving circuit having only a symbol interleaving stage and a tone interleaving stage is degraded.